Ask passengers or airline experts or even Miss Manners about modern airline travel and a common theme emerges: too little space for too many people. The result is squashed laptops, spilled drinks and when disputes become especially tense, flights diverted on their way to their destinations.

Even as passengers grumble about uncomfortable trips, more space does not seem to be in the offing. Though some U.S. airlines are now the world’s most profitable, extra leg room often comes with a fee.

The first of the three recent altercations, all of which ended in planes being diverted, occurred on a United Airlines flight out of Newark, N.J., on Aug. 24. A passenger used a gadget called a Knee Defender to prevent the woman seated in front of him from reclining her seat.

The likelihood of conflict increases when airlines cram more people into a confined space, Sara Nelson, the president of the Association of Flight Attendants union, told NBCNews.com.

And as the airlines squeeze more seats into their planes, seats have become narrower and closer together, says Kathleen Robinette, who worked for 30 years at the U.S. Air Force Research Lab and is now head of the Design, Housing and Merchandising Department at Oklahoma State University.

The minimum seat measurements are typically based on the 95th percentile of men, leaving one in 20 too large for the distance between seats, she said.

Plus, the body measurements being used are the wrong ones, Robinette said. Women are usually wider across the hips than men, and both women and men are wider across the shoulders, she said. The single most important factor for comfort in any seat is the ability to move, she said.

“Virtually everybody on the plane is shoulder to shoulder -- a lot of people with someone else's arm basically in your lap,” she said.

Should Seats Recline?

Until now there has been no incentive for airlines to make seats wider or farther apart, just the opposite, she said. The more seats an airline gets on a plane, the less they need to charge per seat.

Some airlines have seats that do not recline, Spirit Airlines among them, a solution popular with some passengers. In a survey from the travel Web site Skyscanner, 91 percent of travelers said seat reclining should be banned or at least allowed only during set periods on short fights.

But Robinette does not think such bans are the solution. Rather, regulations are needed to ensure more room, she said.

“They’re still selling the tickets for the seats, but consumers are starting to revolt,” she said.

Matt Miller, a spokesman for American Airlines, said that although the airline was retrofitting its aircraft to increase the number of seats on board, the space from one row to another was not shrinking.

“There’s not less leg room per se because of the retrofits that we are working on currently,” Miller said.

He also said it was rare for the airline to divert a flight because of a disruptive passenger.

United Airlines and Delta Air Lines did not immediately respond with a comment.

Defending the Knee Defender

The Federal Aviation Administration does not prohibit the use of the Knee Defender, though all major U.S. airlines say they do.

“That’s their problem,” said the gadget’s inventor, Ira Goldman, whose company, Gadget Duck, has been in business for 11 years. “It’s a customer service issue.”

If airlines do not protect passengers from being battered, the Knee Defender will, the company’s website says. Business is up since the first dispute was reported last week, Goldman said, but he declined to say by how much.

“I’d rather have plastic stop your seat than my knee cap stop your seat,” Goldman said. “That’s why I came up with it.”

Airlines could solve the problem without reducing capacity by installing seats that move forward when they recline, he said.

According to a survey from TripAdvisor, 73 percent of the 4,300 respondents reported uncomfortable seats and limited leg room as their top complaint. Costly airline fees and ticket prices came in second at 66 percent.

As far as the etiquette concerned, Miss Manners has weighed in, suggesting in a 2004 column in The Washington Post that a compromise providing comfort for everyone would be appropriate, reclining only part way for example.

“The real culprit here is the airlines, who install their seats so closely together that the reasonable attitude of reclining a seat that is designed to recline constitutes a nuisance to the passenger behind,” wrote Miss Manners, a pen name for Judith Martin. “However, this deeper problem, of setting minimal comfort standards -- or even minimal health conditions -- for long-haul flights, is not one that etiquette can solve.”